This invention concerns a main injector which increases the weft thread tensioning force, for use in airjet weaving machines.
Such a main injector is mounted in front of the shed, with the purpose of injecting the pick into the shed with as high a velocity as possible.
More particularly, the present invention concerns a main injector comprising a main injector tube extending from the main injector, a hollow threading needle and a mixing zone inside the main injector, and a compressed air supply.
The aim of this invention is to achieve as great a weft thread force as possible. In the prior art this is normally done by increasing the air injection pressure and/or increasing the length of the tube and/or decreasing the tube diameter.
However, all of these methods have disadvantages caused by phenomena associated with the flow of air in the tube.
Regions of fluid turbulence are produced in the tube due to the friction between the fluid medium and the tube wall and/or the internal friction within the medium itself and/or the friction between the medium and the thread being injected. This turbulence, which mainly occurs in the region adjacent the tube wall, has the effect of reducing the effective tube diameter, i.e., the diameter of the region at the center of the tube in which there is a well-defined undisturbed airflow.
Another phenomenon associated with main injectors of the prior art is that the airflow is forced to follow the same path as that of the thread, so that the tensioning force exerted on the thread is greater than would be the case if no tube was used. This favorable phenomenon can be accentuated by increasing the length of the injector tube.
However, because the amount of friction between the fluid medium and the tube wall increases with the length of the tube, so also does the amount of fluid turbulence increase, thus unfavorably affecting the tensioning force on the thread and cancelling out any advantage which might be obtained by having a longer tube.
Another known phenomenon associated with main injectors of the prior art is that the transmission of force to the thread is greater with a smaller tube diameter than with a larger diameter. Because this favorable effect also disappears when the tube diameter becomes so small that the fluid turbulence produced in the region adjacent the tube wall reduces the effective tube diameter to the extent that there is no longer a well-defined, undisturbed airflow, the minimum permissible tube diameter is determined by the size of the turbulent regions and the thickness of the thickest weft thread to be inserted.
It is therefore very important for the thread to be positioned as centrally as possible within the well-defined airflow at the center of the tube and for there to be as little turbulence as possible in the region adjacent the tube wall and to be able to increase the length of the tube and decrease the diameter without the negative effects on pick injection, and to obtain greater tensioning force with the main injector.
Finally, the length of the main injector tube on airjet machines is normally determined by the amount of space available within the construction.